Cascade control system

ABSTRACT

A pneumatically operated cascade control system comprising a primary controller producing an output signal as the set point for a cascaded secondary controller which controls the setting of a process valve. The secondary controller incorporates planar pneumatic technology wherein a thin sheet or plate of metal is formed with openings to define pressure-receptive segments which are pivotable about pivot axes at selected regions. This plate forms part of a laminar sandwich having wall means which cooperate with the plate segments to define pressure chambers to some of which various pressure signals are directed to cause the plate segments to pivot about their pivot axes. Output pressure signals are derived from other of the pressure chambers. The cascade control system comprises transfer switching means for transferring between automatic and manual modes in a bumpless, balanceless fashion. All of the controls required by the process operator are available at the front panel of the instrument, while additional controls for the specialized purposes of maintenance personnel are located in the interior of the instrument with restricted availability.

@Htii States Patent 1 Prescott CASCADE CONTROL SYSTEM [75] Inventor:Robert C. Prescott, Foxboro, Mass.

[73] Assignee: The Foxboro Company, Foxboro,

Mass.

[22] Filed: July 19, 1971 [21] Appl. No.: 163,951

[52] US. Cl "137/14, I M/D19 1, 137/82, 137/84, 137/86, 236/82 [51] Int.Cl ..Fl5b 5/00 [58] Field of Search ..137/DIG. 1, 14, 84, 86; 236/82[56] References Cited UNITED STATES PATENTS 2,969,080 1/1961 Mamzic..137/84 3,126,903 3/1964 Hart et al. ..l37/84 Primary Examiner-AlanCohan Assistant Examiner-Gerald A. Michalsky Att0rneyBryan, Parmelee,Johnson 8!. Bollinger [ill 3,717,162

1 Feb.20,1973

[5 7] ABSTRACT A pneumatically operated cascade control systemcomprising a primary controller producing an output signal as the setpoint for a cascaded secondary controller which controls the setting ofa process valve. The secondary controller incorporates planar pneumatictechnology wherein a thin sheet or plate of metal is formed withopenings to define pressurereceptive segments which are pivotable aboutpivot axes at selected regions. This plate forms part of a laminarsandwich having wall means which cooperate with the plate segments todefine pressure chambers to some of which various pressure signals aredirected to cause the plate segments to pivot about their pivot axes.Output pressure signals are derived from other of the pressure chambers.The cascade control system comprises transfer switching means fortransferring between automatic and manual modes in a bumpless,balanceless fashion. All of the controls required by the processoperator are available at the front panel of the instrument, whileadditional controls for the specialized purposes of maintenancepersonnel are located in the interior of the instrument with restrictedavailabili- 26 Claims, 15 Drawing Figures PATENTEI] FEB 2 0 I975 SHEET30F 6 AC| I PATENTEB FEUZO [U75 SHEET L 0F 6 SECONDARY CONTROLLER ilooCASCADE CONTROL SYSTEM BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to industrial process instrumentation,and particularly to control apparatus of the type commonly referred toas cascade control systems. Such cascade control apparatus consistsbasically of two separate controllers, typically referred to respecloheat-exchanger, while the secondary controller might receive a flowmeasurement signal representing the fuel flow rate into the burnersupplying heat to the exchanger.

The purpose of a cascade control system is to maintain the primaryvariable (e.g. output temperature) at a set level. To this end, theoutput of the primary controller furnishes the set point signal for thesecondary controller. Incorporation of the cascaded secondary controllerin the control loop provides improved performance in achieving thisresult.

2. Description of the Prior Art Cascade control systems have been incommercial use for many years, in a variety of forms. One seriousproblem with cascade control systems is that of switching betweenautomatic and manual modes of operation, in a bumpless, balancelessfashion, i.e. without requiring any intermediate balancing operationduring the switchover procedure. The problem of switching from manual toautomatic operation has presented a particularly difficult problem dueto the complexity of the overall system, i.e. wherein there are twoseparate controllers each of which must be prevented from introducingany upset or bump in the process at the time of switchback to automaticmode.

It has been found preferable to mount the primary and secondarycontrollers as a physically integrated unit, i.e. as a so-calledsingle-station cascade controller. A practical difficulty in achievingthis result in a pneumatic control system stems from the fact thatordinary pneumatic controllers are relatively large in size, so that theoverall package which includes two separate conventional pneumaticcontrollers can be excessively bulky for modern control stationarrangements.

SUMMARY OF THE INVENTION In the preferred embodiment to be describedhereinbelow in detail, a cascade control system of the pneumatic type isprovided wherein both the primary and secondary controllers are mountedtogether as a singlestation cascade controller. The primary controllerin this embodiment is essentially a known pneumatic controller havingconstructional arrangements and operational capabilities similar topreviously used equipment such as is disclosed in US. Pat. Nos.2,724,917 and 3,515,162. This primary controller produces an outputpressure signal which is directed to the secondary controller as the setpoint signal for that controller. The specific primary controllerdisclosed is of the threemode type, including proportioning, reset andrate functions, but it will be apparent that the system will operatewith other types of primary controllers. The secondary controllerdisclosed is a two-mode unit, providing proportioning and resetfunctions, but other types of secondary controllers also can be used inthe system of the present invention.

When the system is conditioned for manual operation, the valve signal iscontrollable directly from the front panel of the station, and both theprimary and secondary controllers are effectively eliminated from thecontrolling function. The transfer switching arrangement for making theshift from automatic to manual operation is similar in importantconceptual respects to the apparatus disclosed in co-pending applicationSer. No. 067,750 filed by Earl A. Beardsley et al. on Sept. 1, 1970, andaccordingly information on certain details of the present apparatus canbe obtained by reference to that co-pending application. In brief,however, the present equipment includes a pneumaticallyoperated servoposition-follower, which, while the system is in automatic mode,continuously adjusts the manual signal control lever to a positioncorresponding to the automatic valve signal being directed to theprocess valve. Thus, upon switchover from automatic to manual, themanual unit will produce a valve signal exactly equal to the previouslyexisting automatic control signal, thereby avoiding any bump of theprocess valve.

For switchback from manual mode to automatic mode, a distinctivearrangement is used to assure bumpless transfer, still without requiringany intermediate operational procedures such as a balancing step. Inmore detail, while the system is in manual mode, it is maintained inreadiness for one-step transfer back to automatic mode by means which(1) continuously holds the output of the secondary controller equal tothe manual signal, and (2) simultaneously maintains the primary andsecondary controllers in appropriate conditions to avoid upsetting theprocess upon switchback to automatic, and accomplishes this withoutrequiring a primary set-point change.

In the preferred embodiment, this latter result is achieved in part bycontinuously adjusting the primary reset pressure to maintain it at avalue which produces a primary output signal equal to the secondarymeasurement signal, and by continuously adjusting the secondary resetpressure (through a non-restrictive, bypass channel) to be equal to themanual valve signal. Withthis arrangement, the primary and secondarycontrollers will be continuously maintained in condition to avoidupsetting the process both during and after switchback to automaticmode.

In effect, the primary controller will, when the system is in manualmode, be conditioned to call for the secondary variable magnitude (e.g.flow rate) which is currently being produced by the manually-set valveposition. Thus the primary controller conditions the secondarycontroller so that it can call for the actual existing valve position.

After switchback to automatic, the control system will, if there is anydeviation between the primary measurement and set signals (as there islikely to be), cause the valve to be repositioned to whatever extentmight be necessary to bring the primary measurement signal back to theprimary set point. This latter control action will however be effectedsmoothly, without a step change due to proportioning action, and theresponse dynamics will be in accordance with the already establishedcontrol settings of proportioning, derivative and reset.

The secondary controller, in the preferred embodiment to be described,is based upon certain aspects of the planar pneumatic technology whichhas been disclosed in 11.8. Pat. Nos. 3,590,694 and 3,593,734. Suchplanar technology has, however, been further extended in this newcontroller by novel features to be described hereinbelow. The resultingcontroller provides advanced operational capabilities yet is quite smallrelative to conventional controllers. Such features have contributedsignificantly to the achievement of a commercially practical,single-station pneumatic cascade system combining both the primary andsecondary controllers in a unitary instrument casing of standarddimensions.

Accordingly, it is a principal object of the present invention toprovide improved apparatus and methods suited for controlling industrialprocesses. A more specific object of the invention is to provide asuperior cascade control system having unique means for transferringfrom manual to automatic operation. Still another object of theinvention is to provide improved pneumatic signal-handling means of theplanar type, capable of extending the capabilities of such apparatusinto new areas. Other objects, aspects and advantages of the inventionwill in part be pointed out in, and in part apparent from, the followingdescription considered together with the accompanying drawings in which:

FIG. 1 is a perspective view, partly cutaway, of a single-stationpneumatic cascade controller in accordance with this invention;

FIGS. 2A, 2B and 2C together present a schematic diagram showing theoverall pneumatic circuit of the cascade control system, partly inperspective, and partly pictorial;

FIG. 2D shows the manner in which FIGS. 2A, 2B and 2C are joined topresent the complete circuit;

FIGS. 3A through 3] show in perspective individual layers of the planarpneumatic construction utilized in the secondary controller; and

FIG. 4 is a partial vertical section showing elements of the planarpneumatic controller of FIGS. 3A through 3I.

Referring now to FIG. 1, the cascade control apparatus comprises aninstrument casing within which is mounted a primary pneumaticcontroller, illustrated partially at 12 as a force-balance unit of knownconstruction, together with a secondary pneumatic controller 14 ofunique construction to be described hereinbelow. The primary controlleris of the so-called three-mode type, providing the control functionsreferred to as proportioning, reset and rate. The secondary controlleris a two-mode type, providing proportioning and reset.

At the front of the instrument there is an indicator scale 16 havingthree separate pointers 18, 20, 22. One of these pointers 18 serves toindicate the primary set point level (e.g. a desired temperature), andis manually adjustable from the front panel by means of the usual knob24. The second pointer 20 indicates the actual measured value of theprimary condition, for ready comparison with the set point level.

The third pointer 22, extending in from the left-hand side of the frontpanel scale, normally indicates the measured value of the secondarycondition (e.g. rate of fuel flow into a heat exchanger). However, foroperator convenience, this pointer can alternatively be used to indicatethe value of the secondary controller set point (i.e. the output of theprimary controller). A two-position knob 30 is provided at the frontpanel to permit the operator to shift the indicator between these twoshared functions.

FIGS. 2A, 2B and 2C when joined together as out lined in FIG. 2D, showin schematic circuit form the overall arrangement of the cascade controlsystem.

Referring first to FIG. 2A, the primary controller 12 includes aninitial derivative or rate unit 40 of the force-balance type whichreceives from a conduit 42 the primary measurement signal in the form ofa pneumatic pressure representing the magnitude of the primary conditionto be controlled. Such primary measurement signal is produced by anyconventional measurement instrument of transducer, such as a temperaturemeasuring instrument illustrated at 44 in association with an exemplaryheat-exchanger process 46. This measurement signal in conduit 42 also isdirected through another conduit 48 and a transfer valve 50 to apneumatic bellows-type receiver 52 of known construction. This receiveractuates an output linkage 54 which positions the measurement pointer 20(see also FIG. 1) at the front panel of the instrument.

The output signal from the derivative unit 40 is directed through aconduit 60 to the input or measurement bellows 62 of a four-bellowspneumatic forcebalance unit 64 of known construction, illustrated (forsimplicity) with a balance beam 66 pivoted at 68. This beam is arrangedto receive at its right-hand end the forces developed by the measurementbellows 62 and an opposed set point bellows 70. This set bellows 70receives a set point pressure signal from conduits 72 and 73 leading toa set-point transmitter 74 (FIG. 2B) of known construction,controlled'by the front panel knob 24 previously referred to.

Any unbalance in the forces applied to the balanceable beam 66 by themeasurement and set bellows is detected by the usual pneumatic nozzle 76supplied with air through a flow restrictor 78, such as the aspiratorarrangement described in US. Pat. No. 3,574,486.

As is well known, the back pressure of this nozzle 76 is a function ofthe position of the beam 66, and this pressure is supplied directlythrough a conduit 80 to a negative feedback bellows 82 which applies acorresponding force to the left-hand end of the beam. The nozzle backpressure also is directed through a pneumatic reset circuit which, inthis embodiment, includes a conduit 84, a manually-operable transferswitch 86 (the function of which will be discussed later), conduits 88and 90, a pneumatically-controllable close-off switch 94 (the functionof which also will be discussed later), an adjustable reset flowrestrictor 96, and a reset tank or volume 97 connected to a resetbellows 98. This latter bellows is positioned to oppose the feedbackbellows 82, and produces reset action in the controller output signal inknown fashion. The nozzle back pressure in conduits 84, 88 serves as theoutput signal of the primary controller 12 and is directed through anoutput conduit 100 to the secondary controller 14 (FIG. 2C) to providethe set signal for that controller. Thus the primary controller outputsignal is in effect an intermediate output signal of the cascade controlsystem.

The secondary controller 14 also receives from another input conduit 102a pressure signal representing the secondary measurement (e.g. fuel flowrate, as in the illustrated example) developed by a conventionalmeasuring instrument 104. The secondary controller compares these twopressure signals, in a manner to be explained, and produces anappropriate output pressure signal in a conduit 106 leading to an outputrelay 108 (FIG. 2C). The relatively high-powered signal of this relay isutilized to produce feedback action in the secondary controller, andalso provides a final automatic output signal which is directed from therelay output conduit 109 and connecting conduits to the process valve110 (FIG. 2A). In this embodiment, the

valve is illustrated as a fuel flow control valve for the heat-exchangerprocess 46. (Although an actual valve is illustrated in connection withthis embodiment, it is to be understood that the output signal can beused to manipulate any variable, operator, or process influencin gelement, and that the term va1ve" therefore should be understood tobroadly embrace such other types of elements.)

THE SECONDARY CONTROLLER The secondary controller 14 incorporates, andis based upon, what has sometimes been referred to as planar pneumatictechnology. The fundamental concepts of such technology have beendescribed in detail in U.S. Pat. Nos. 3,590,694 and 3,593,734. Brieflyspeaking, planar pneumatic devices are multi-layered sandwich structuresthe operating elements of which typically comprise thin metal platespivotally or hingedly mounted for movement responsive to applied fluidpressures. The movements of such plates serve to control or regulatevarious pneumatic signals or the like.

The secondary controller 141 is of such a laminar construction, and theindividual layers of the present embodiment are shown separately inexploded perspective in FIGS. 3A through 3I. However, to simplify thepresentation and explanation of the functional operation of thecontroller, the schematic illustration of FIG. 2C shows only the flexureassembly portion 200 of the structure. This structure consists of a pairof somewhat springy but essentially form-retaining matched thin metalsheets or plates 202A, 2028 provided with aligned cut-out sections oropenings so as to define a multi-segmented pivotable operator 204 and anelongate element 206 hingedly supported in cantilever fashion atone end206A.

A thin rubber sealing diaphragm 208 is secured between the two metalplates 202A, 20213. This diaphragm extends across the cut-out openingsin the plates to seal off and thereby isolate the pressures on oppositesides of the flexure while permitting small movements of the pivotableoperator 204 about its pivot axis 210, and similarly permitting smallmovements of the hinged element 206 about its cantilever support 206A.Cut-out 207 is provided to fix an appropriate springiness for such smallmovements of element 206.

FIG. 2C also includes pictorial representations of conduits and the liketo indicate the manner in which the various parts of the controllercooperate and co-act to produce the final automatic output controlsignal. Not shown in FIG. 2C are the various other layers which adjoinand effectively surround the flexure assembly 200. These additionallayers together serve to define sealed pressure chambers adjacentspecific portions of the operator 204 and the element 206, to sealinglysupport the operator for pivotal movement about the pivot axis 210, andalso to form numerous interconnecting passages for conducting pneumaticpressure signals between the pressure chambers and various operatingparts.

The pivotable operator 204 comprises a relatively large-area tongue-likesegment 212 below the pivot axis 210, and two smaller separatetongue-like segments 214, 216 above the pivot axis. An isolation cutout215A in lower segment 212 effectively avoids rigid connection betweenoperator 204 and the adjoining outer section 21158 of the flexure plates202A, 2028, and thus accommodates the required pivotal movement of theoperator. Adjoining layers of the laminar structure (referring now alsoto FIG. 4) define one pair of pressure chambers 218, 220 adjacent therespective opposite surfaces of the lower tongue 2112, another pair ofpressure chambers 222, 224 adjacent the respective opposite surfaces ofthe right-hand upper segment 216, and a third pair of pressure chambers226, 228 adjacent the respective opposite surfaces of the hinged element206.

The flexible sealing diaphragm 208 prevents air flow through the cut-outregions around the operator 204 and element 206. Thus the opposingpressure chambers are effectively sealed to permit independent pressurevariations therein. The left-hand upper segment 214 extends out throughthe sealed pivot region 210 to beyond the outside walls of the laminarstructure, i.e. into open space, typically atmospheric pressure. Thesealing diaphragm 208 does not extend out into that area.

The pivotable operator 204 is a force-balance device; that is, thetorques about the pivot axis 210 are maintained in balance by negativefeedback action. The feedback signals are developed basically by apneumatic position-sensing nozzle 240 associated with the externaloperator segment 214 and connected through output conduit I06 and arestrictor 242 to the usual air supply source, e.g. at 20 psi. Inessence, the nozzle 240 detects any unbalance in the total torque aboutaxis 210 resulting from differences between the measurement and setpressures applied to opposite sides of the right-hand operator segment216 (in the respective pressure chambers 222, 224), and between thepressures applied to opposite sides of the lower operator segment 212(in the respective pressure chambers 218, 220). As noted above, the setpressure signal is derived from the primary controller, as thesocalled"intermediate output signal of the cascade control system, whilethe measurement pressure signal is derived from a conventional measuringinstrument 104. The pressures in chambers 218, 220 are controlled bypneumatic feedback circuits, to be described, responsive to theposition-sensing nozzle 240.

The back-pressure of position-sensing nozzle 240 is directed throughconduit 106 to the output relay 108 to produce a corresponding relaysignal which serves to activate feedback circuits mentioned above andalso serves as the final automatic control signal of the control system,providing valve positions appropriate for maintaining the secondarymeasurement and set signals equal. When the process is stabilized withmeasurement and set pressures equal, the torques on the pivotableoperator 204 will be balanced with the operator positioned such that thenozzle back-pressure will produce the correct control signal formaintaining the valve position.

If the secondary measurement and set signals become unequal, theresulting pressure differential between the opposite surfaces of theupper operator segment 216 will develop a torque about the pivot axis210. This torque will tend to rotate the pivotable operator, causingcorresponding movement of both the lower tongue 212 and the externalsegment 214. That is, all of the tongue segments are integral and thusmove together about the pivot axis. Consequently, the backpressure ofnozzle 240 will change, so as to alter correspondingly the outputpressure from the output relay 108.

This relay output pressure is conducted through a spool valve 250 (thefunction of which pertains primarily to the automatic-to-manual transferarrangements to be discussed below) to a negative feedback conduit 252leading to the pressure chamber 220 behind the lower tongue 212. Thisnegative feedback circuit serves to maintain the net torques on thepivoted operator 204 in balance. When the torque is momentarilyunbalanced by a change in measurement or set pressures, the resultingchange in relay output pressure produces a corresponding change in thefeedback pressure in chamber 220 just sufficient to counteract theinitial torque unbalance. When balanced conditions are restored, the newoutput pressure from relay 108 represents the secondary controlleroutput signal for appropriately readjusting the process valve to bringthe measurement signal into equality with the set signal.

This secondary controller 14 also includes means for developing a resetfunction in its output signal. In effect, the control action (opening orclosing of the valve) is gradually augmented in response to a persistentdeviation between measurement and set signals. This reset is produced bya positive feedback circuit wherein the relay output pressure isconducted through spool-valve 250 and a positive feedback conduit 254 toa variable pneumatic reset restrictor 256. (This restrictor isillustrated as part of a dual restrictor assembly 255, and is presentedin the form of a so-called scratch restrictor of known design which willbe referred to in somewhat more detail hereinbelow.) From the output256A of the reset restrictor 256, the reset circuit continues through arelatively largecapacity tank or volume 257 and a damping restrictor 258to the lower pressure chamber 228 behind the hinged element 206.

This hinged element 206 is utilized primarily for pneumatic isolation,and serves as a one-to-one (1:1) repeater to produce in the frontchamber 226 a pressure equal to the reset pressure in chamber 228. Forthis purpose, the front chamber 226 is connected through a fixedrestrictor 260 to a supply of air under pressure (e.g. 20 psi), and avent nozzle 262 is mounted in the chamber adjacent element 206 so as toadjust the degree of vent restriction in accordance with the position ofthe element. If the pressure in the front chamber 226 becomes unequal tothe reset pressure in the rear chamber 228, the element 206 will bemoved by the resulting pressure differential so as to alter theeffective restriction presented by the vent nozzle, and thereby restorebalanced pressure conditions. Thus the pressure in chamber 226 will bemaintained equal to the reset pressure in the opposite chamber 228.

The repeated reset pressure is coupled from chamber 226 through apressure dropping restrictor 270 (the function of which will bedescribed later) and a damping restrictor 272 to the pressure chamber218 in front of the lower tongue 212 of the pivotable operator 204. Thepressure in chamber 218 tends to counteract or reduce the negativefeedback torque produced by the pressure in the rear chamber 220, andthus can be considered as providing a degree of positive feedback. Thedynamics of the positive feedback action depends, as in conventionalpneumatic reset circuits, on the setting of the reset restrictor 256, aswell as on the magnitudes of other fixed parameters of the resetcircuit. At all times, however, the torques on the pivotable operator204 are maintained in balance by the negative feedback action from thesensing nozzle 240.

A process controller should for most applications have means foraltering its so-called proportioning band. That is, there should bemeans for adjusting the amount of change in the final control signal(excluding time-variant rate or reset effects) for a given change indeviation between measurement and set. In conventional force-balancepneumatic controllers, this adjustment typically is accomplished bychanging the effective location of the pivot point of the force-balancebeam. However, with controllers based on planar pneumatic technology,such as illustrated by the secondary controller 14 herein, such anadjustment of the pivot point position is not truly practical, due tothe nature of the laminar sandwich construction of a planar device.Thus, providing a suitable adjustment of the proportioning band hasposed a serious problem.

This problem has been solved in accordance with one aspect of thepresent invention by employing a variable pneumatic restrictor tocontrol the degree of response of the controller to a given change indeviation signal. In the preferred embodiment, this proportioningrestrictor is incorporated in a circuit leading from the output relaypressure to the positive feedback pressure chamber 218, and serves ineffect to control the degree of positive feedback counteraction opposingthe negative feedback pressure in the negative feedback pressure chamber220. That is, this proportioning positive feedback effectively cancels aproportion of the negative feedback torque in accordance with thesetting of the proportioning restrictor.

More specifically, the dual restrictor assembly 255 includes a secondadjustable scratch restrictor 280- which is connected, by a conduit 282,between the relay output pressure and the fixed pressure-droppingrestrictor 270 leading to the repeater output chamber 226. Thus thissecond restrictor 280 is effectively in series with the fixed restrictor270, and the two together serve functionally as a pressure-dividingnetwork, providing at the juncture 284 between the two restrictors apressure having a magnitude somewhere between the magnitude of theoutput and reset pressures. The actual magnitude of this positivefeedback pressure is a function of the ratio of the fixed and variablereset restrictors 270 and 280. This positive feedback pressure isdirected through the damping restrictor 272 to the positive feedbackchamber 218. Thus it will be evident that the degree of proportioningaction is controlled by the setting of the adjustable restrictor 200.

To summarize, the net amount of positive feedback tending to counteractor cancel out the negative feedback in chamber 220 will be determined,in part, by the setting of the second scratch resistor 280. The resultis that an adjustment of the setting of that restrictor varies theeffective amount of negative feedback, and thereby determines themagnitude of the output pressure from the relay 108 for a givendeviation between the secondary measurement and set pressures. In thatmanner, the adjustable restrictor 280 serves to set the proportioningband of the controller 14.

Advantageously, the fixed pneumatic restrictors associated with theplanar pneumatic controller 14 are small holes formed in the flexureassembly 200. Thus, in FIGS. 3C, 3D and 3E, aligned holes are shown asproviding the restrictors identified with the reference numbers used inother portions of this specification. The manner in which suchrestrictor holes are connected into the associated pneumatic circuits isas described in US. Pat. No. 3,593,734. Also in accordance with thatpatented technology, use is made of pneumatic passages formed in thelaminar sandwich castings, shown in FIGS. 3A and 3G, for purposes ofeffecting desired interconnections as outlined in the circuit diagram ofFIGS. 2A, 2B and 2C. The casting of FIG. 3A also is formed with anenlarged portion 257A to serve as the reset volume 257 associated withthe adjustable scratch resistor 256 for setting the degree of resetaction.

The restrictor assembly 255 (FIG. 2C) comprises a pair of plastic discs290A, 290B each having a tapered groove 292A, 292B formed on one facewhich is pressed against a Teflon sealing plate 294 containing outletopenings 256A, 280A aligned with the corresponding groove. Therotational position of each of the discs is individually controllable bymanually operable means (not shown) so as to permit altering therelative positioning of the tapered groove with respect to the outletopening. Each disc also is formed at the large end of its tapered groovewith a corresponding passage 296A, 296B which connects through to theinterior 298 of the restrictor assembly chamber. This chamber interioris pressurized by the output pressure from the relay 108. As either discis rotated, there is a corresponding change in the length of the taperedgroove through which the output air pressure must pass to reach thecorresponding outlet opening. By these means, the amount of restrictionor pneumatic impedance placed in the path of the corresponding air flowcan be varied, so as to alter the proportioning band or the reset of thecontroller.

BUMIPLESS TRANSFER BETWEEN MANUAL AND AUTOMATIC In accordance withanother aspect of the present invention the control apparatus includesmeans for transferring between manual operation and automatic operationin a bumpless fashion (i.e. without upsetting the process), and withoutrequiring any balancing or other procedure to prepare the system fortransfer. When in manual mode, the pneumatic valve signal is produced bya manual unit 300 (FIG. 2A) physically located beneath the instrumentcasing 10 (FIG. I). This manual unit is of known construction, as shownfor example in US. Pat. No. 3,525,351. The manual unit 300 includes amain transfer switch in the form of a rotary valve 302, operated fromthe front panel of the instrument by a lever 304 having automatic andmanual positions as indicated by the labels in FIG. 1.

When the system is in automatic mode (with the primary and secondarycontrollers operating as described previously), the manual unit 300tracks the automatic signal from output relay 108, and continuouslyadjusts the positioning of a manual signal control lever 306 tocorrespond with the actual automatic output signal. Thus, when thesystem is switched from automatic to manual (where the ports of thetransfer valve 302 will be aligned as shown in dotted outline in FIG.2A), the manual signal produced by unit 300 will be equal (withinappropriate tolerances) to the previously existing automatic outputsignal. This manual signal is directed through a conduit 308 to thevalve 110, in place of the automatic output signal produced by relay108. The magnitude of the manual signal can thereafter be altered atwill by rotating a thumb wheel 310 one edge of which extends through thefront panel of the instrument 10 (FIG. 1), and which is shifted intodriving engagement with the arc element 312 of lever 306 when thetransfer switch 302 is rotated to manual position.

When the transfer switch 302 has been placed in its manual position, itsends an air pressure switching signal (e.g. 20 psi) out through aconduit 320 to operate various switching elements, now to be described,so as to prepare the control system for transfer back to automatic modein a bumpless, balanceless fashion. One such switching element is theon-off switch 94 previously referred to. This switch is of conventionalconstruction including a flexible diaphragm 324 shiftable by thepressure of the pneumatic switching signal into a position shutting offcommunication between the two ports 326, 328. When so actuated by theswitching signal, in manual mode, this switch 94 closes off the resetfeedback circuit of the primary controller 12. Thus, when in manual modethe reset pressure in reset bellows 98 will not varied in correspondencewith the output pressure from the primary controller, but instead (aswill be explained) is adjusted in response to changes in the secondarymeasurement signal in such fashion as to maintain the primary controlleroutput pressure equal to the secondary measurement pressure.

The switching signal from conduit 320 also actuates the spool valve 250to its right-hand position, illustrated in light outline. In thatposition, the spool valve interrupts the connection between the outputrelay 108 and the process valve 110, while continuing the connectionfrom the output relay to the negative feedback chamber 220 of thesecondary controller 14. Also, in the right-hand (manual mode) position,the spool valve interrupts an air supply connection to a conduit 340leading to two conventional diaphragm-type pneumatic switches 342 (FIG.2B) and 344 (FIG. 2C). In the preceding automatic mode condition, theseswitches were maintained closed by the effect of the air supply pressureon the corresponding diaphragms, but when the supply air is interrupted,in manual mode, these switches are opened.

CONDITIONING OF PRIMARY CONTROLLER IN MANUAL MODE The first pneumaticswitch 342, when opened at transfer to manual mode, completes a directconnection between the primary controller reset bellows 98 (FIG. 2A) andthe output chamber 346 of a forcebalance planar transfer unit 348 ofknown construction (see, for example, the above-mentioned US. Pat. No.3,590,694. This transfer unit includes. a pivotable operator 350arranged to compare continuously the secondary measurement signalpressure, in one upper input chamber 352, with the primary controlleroutput signal pressure, in the other upper input chamber 354. Thesesignal pressures are conducted to the corresponding chambers byrespective conduits 356, 358. The transfer unit is activated in manualmode by the supply air pressure which also actuates the spool valve 250.

If, now, the primary controller output pressure directed to transferunit chamber 354 becomes unequal to the secondary measurement pressuredirected to the opposing chamber 352, the output of the transfer unit,from its chamber 346, correspondingly changes to readjust the pressurein the primary controller reset bellows 98 (FIG. 2A) to force theprimary controller output pressure into equality with the secondarymeasurement signal. This readjustment is effected by negative feedbackaction initiated by the transfer unit vent nozzle 360 which senses anyslight change in positioning of the operator 350 resulting from aninequality between the pressures in opposed input chambers 352, 354.

More specifically, the chamber 362 in which this vent nozzle 360 islocated is supplied with air through a restrictor 364 so that any changein vent nozzle air flow due to a change in position of operator 350correspondingly alters the pressure in chamber 362 to tend to maintainthe net torques on operator 350 in balance. This vent chamber 362 isconnected through a restrictor 366 to the transfer unit output chamber346, and also to the primary controller reset tank 97 and reset bellows98. The combination of restrictor 366 and reset tank 97 produces aneffective integrating action of the air pressure signal developed inchamber 362, while the nozzle 360 continues to adjust the pressure inchamber 362 such that the net torques on operator 350 remain in balance.This integrating action provides appropriate stability for the negativefeedback circuit now to be described.

The change in the transfer unit output pressure .(chamber 346)transmitted to the reset bellows 98 tends to unbalance the primary forcebeam 66. This, in

turn, causes the primary sensing nozzle 76 correspondingly to readjustthe output pressure of the primary controller, and thereby alter thepressure in the transfer unit input chamber 354. This change in pressure is in a direction tending to restore equality with the secondarymeasurement pressure in the opposing transfer unit chamber 352. Thus,the overall circuit provides negative feedback action which continuouslymaintains the primary controller output pressure equal to the secondarymeasurement signal.

CONDITIONING OF SECONDARY CONTROLLER IN MANUAL MODE The second pneumaticswitch 344 (FIG. 2C), referred to above, is opened by the air pressuresignal from conduit 340 upon transfer to manual mode by transfer switch302. Opening of this switch completes a connection from the secondarycontroller output conduit 254 to the secondary reset volume 257, andthence to the reset pressure chamber 228. This connection thuseffectively by-passes the reset restrictor 256. (Although this by-passconnection includes a damping restrictor 380, its restrictive effect isessentially negligible relative to the reset restrictor 256.)Accordingly, the reset pressure in chamber 228, and in the adjacentrepeater chamber 226, is maintained equal to the manually-set pressurebeing transmitted to the process valve 110. Since only a dampingrestrictor 380 is in the by-pass circuit, the reset pressure will followclosely any changes in manual pressure.

The manually-set valve pressure present in conduit 254 also is directedthrough the proportioning restrictor 280 to the pressure-dividerrestrictor 270 leading to the repeater chamber 226. Since the pressurein the repeater chamber is equal to the manually-set valve pressure, itwill be evident that the pressure at restrictor juncture 284, and thusthe pressure in the positive feedback chamber 218, necessarily will bemaintained equal to the manually-set valve pressure. The proportioningrestrictor 280 will not prevent this pressure from closely following themanually-set pressure, because the proportioning restrictor has asubstantial air-flow capacity, significantly greater than that of thereset restrictor 256.

As noted previously, the output pressure of the primary controller 12(conduit is continuously maintained equal to the secondary measurementpressure (conduit 102). Thus, the pressures in the opposed inputchambers 222 and 224 of the secondary controller will necessarily beequal, so that no net torque is applied to the pivotable operator 204 bythese chambers. Accordingly, the feedback action of the sensing nozzle240 adjusts the pressure in the negative feedback chamber 220 to beequal to the pressure in the positive feedback chamber 218, which inturn is equal to the reset pressure in the repeater chambers 226, 228.

In effect, the secondary controller 12 is maintained in completelybalanced state, with its measurement and set pressures equal, and itsreset and output pressures both equal to the manually-set valvepressure. Thus, when the system is switched back to automatic mode,there will be no immediate change in the pressure fed to the processvalve 110, so that the transfer will be bumpless. The secondarycontroller will not introduce any upset immediately thereafter, becauseits measurement and set inputs 100, 102 are equal, due to conditioningof the primary controller 12, discussed in detail hereinabove. Ofcourse, subsequently the control system will take over automatic controlof the process, and, if there is any deviation between the primarymeasurement and set signals (as there is likely to be), the cascadedcontrollers will bring the measured variable to its desired set point.This control action will be smooth, because the primary controller resetpressure is at the proper level for the conditions then existing, andthe dynamic response will be determined in the usual fashion by thecontroller parameters (proportioning, rate and reset) which have beenpreset in accordance with the particular process being controlled.

TRANSFER FROM CASCADE TO SECONDARY CONTROL At times, e.g. formaintenance and adjustment purposes, it may be desirable to transfer theprocess from full cascade automatic control to, control only by thesecondary controller 14. To effect such transfer, the control systemfirst is switched to full manual control, as described above, byactuation of transfer switch 302 (FIG. 2A) to its manual position. Thusthe process valve 110 is controllable only by the manual unit 300 and isunaffected by any manipulations of the remainder of the controlapparatus.

With the process under manual control, the secondary transfer switch 86is actuated to its secondary position (shown in light outline), byrotation of rod 400 which is connected to a lever (not shown) accessiblefrom the side of the instrument when the instrument has been partiallywithdrawn from its mounting panel. With switch 86 in its secondaryposition, the primary set pressure from conduit 73 is transmittedthrough to conduit 88 and thence to the secondary set point conduit 100leading to the secondary controller 14. The primary set point pressurethen is adjusted, by front panel knob 24, until the set pointer 18matches the secondary measurement pointer 22.

Now the system is conditioned for smooth transfer to secondary controlby returning the transfer switch 302 to automatic position. Thesecondary controller will thereupon pick up automatic control of theprocess valve 110, in the manner explained hereinabove, but with its setpoint controllable by knob 24 rather than by operation of the primarycontroller 12.

(It may be noted that after the system first is switched to manual mode,the order in which the subsequent steps are taken is not important,since the process valve is isolated from the automatic controlapparatus, and is adjustable only by the manual unit 300. This is asafety feature, avoiding the possibility of process damage throughoperator error.)

To return from secondary control to full cascade control, theabove-described procedure is simply reversed. That is, transfer switch302 first is placed in manual position to isolate the process valve fromthe control apparatus. Then the set point knob 24 is adjusted to bringthe set pointer 18 to the desired primary set point, and the secondarytransfer switch 86 is returned to its normal cascade position (asshown). The various conditioning circuits described hereinabove thusbecome operative to prepare both the primary and secondary controllersfor bumpless, balanceless return to cascade control. Accordingly, whenthe transfer switch 302 is returned to automatic position, the system isbrought smoothly back to full cascade control without upsetting theprocess. 4

CONSTRUCTION DETAILS OF THE SECONDARY CONTROLLER FIGS. 3A through 31provide perspective views of the laminar sections which are combined ina sandwich construction to form the secondary controller 14. The centralportion is the flexure assembly 200 which provides the pivotableoperator 204. This flexure assembly is similar to the operator shown inthe above-identified U.S. Pat. No. 3,590,694, and comprises the twoouter sections 202A, 202B of thin springy metal, adhesively secured (asby epoxy) to corresponding sides of the mating rubber sealing diaphragm208. Adjacent opposite sides of the flexure assembly 200 are respectivesealing gaskets 410, 412.

These gaskets 410, 412 furnish required pressure sealing for the topcasting 414, on one side, and the base sandwich 416 on the other side.This base sandwich comprises a chamber casting 418 adhesively secured toa thin cover plate 420 which serves to seal various pneumatic passagesformed in the remote side of the chamber casting to define pressureconduits somewhat in the fashion that such pressure conduits are formedin the pneumatic computing apparatus disclosed in U.S. Pat. No.3,371,862 issued to H. L. Bowditch et al. The closed passages formed inthe base sandwich 416 provide various interconnections for the pneumaticcircuit of the secondary controller, described hereinabove withreference to FIG. 2C, and including passages leading to the numeroussmall holes in the flexure assembly which serve to provide predeterminedrestrictions in selected pneumatic flow paths.

The final section of the laminar sandwich construction is a relativelyrigid base plate 422 provided with access openings to make connectionsbetween the secondary controller 14 and other elements of the cascadecontrol system, in accordance with the circuit diagram of FIGS. 2A, 2B,2C, previously described. These access openings incorporate rubberO-rings inserted therein to seal the connections between the controllerl4 and the pneumatic circuit board (not shown herein in detail) to whichthis controller 14 is secured, in accordance with the techniquesdescribed in copending application Ser. No. 864,108, filed on Oct. 6,l969, by l-loel L. Bowditch, now U.S. Pat. No. 3,631,881.

This planar construction of the secondary controller 14 provides quitehigh sensitivity to input signals, and makes it possible to operate thecontroller with a low powered set point signal taken directly from theaspirator nozzle 76 of the primary controller 12. Thus, it isunnecessary to use a pneumatic relay, or the equivalent, to boost thepower of the set point signal developed by the primary controller.

Although a specific preferred embodiment of the invention has beendescribed in detail, it will be apparent that various modifications canbe made. For example, when the system is on manual operation, thesecondary reset pressure can be maintained equal to the manual valvesignal by the use of a comparison servo arrangement incorporating aplanar transfer unit, i.e. by an arrangement similar to that used tomaintain the primary reset pressure equal to the secondary measurementpressure. For some applications, it may be advantageous to use such aplanar transfer unit arrangement to compare the secondary output andmanual signals, and to continuously adjust the primary reset pressure tomaintain equality between the compared signals.

I claim:

1. in a cascade process control system of the type including a primarycontroller arranged to receive both primary measurement and primary setpoint signals and to provide an intermediate output signal to serve as aset point signal for a secondary controller arranged to receive asecondary measurement signal for comparison with the secondary set pointsignal, said secondary controller serving to provide an automaticcontrol signal to adjust the setting of a process operator element suchas a valve or the like so as to maintain the primary measurement at itsset point value, and wherein the system is transferable betweenautomatic and independent (e.g. manual) control modes and for thatpurpose includes a selectively activatable means operable in independentmode for producing a separate signal, in the form of a manual signal orthe like, for controlling the process operator element in place of saidautomatic control signal;

that improvement in said control system for effecting bumpless,balanceless transfer from independent mode to automatic mode whichcomprises, in combination:

transfer switch means selectively operable into automatic or independentstatus and serving in independent status to:

1. direct to said process operator element a control signalcorresponding to said separate signal;

maintain said secondary controller functioning to produce an automaticoutput signal corresponding to said separate independent signal;

3. maintain said primary controller functioning,

with a constant set point, to produce an intermediate output signalcorresponding to the secondary measurement signal; said transfer switchmeans serving when switched back to automatic status to:

1. direct to the process operator element a control signal correspondingto said automatic output signal produced by said secondary controller(which automatic output signal had during the preceding independent modebeen maintained equal to said separate signal whereby the processoperator element will remain unaffected at the instant of switchback toautomatic mode); and

. release said primary controller from the effective influence of saidsecondary measurement signal and return it to normal operation under theinfluence of said primary measurement and set signal, whereby theprimary controller will not alter the functioning of the secondarycontroller so as to upset the process but will, after switchback toautomatic, readjust the set point of the secondary controller towhatever extent is required for the purpose of automatically controllingthe primary process condition to its set value.

2. Apparatus as claimed in claim 1, wherein said selectively activatablemeans comprises a manual signal unit having means for manuallycontrolling the magnitude of a signal for the process operator element.

3. Apparatus as claimed in claim 1, wherein said primary controllerincludes a reset element to produce reset action in the intermediateoutput signal; and

adjusting means operable in independent mode responsive to saidsecondary measurement signal for adjusting the activation of said resetelement to maintain said intermediate output signal equal to saidsecondary measurement signal.

4. Apparatus as claimed in claim 3, wherein said primary and secondarycontrollers are of the pneumatic force-balance type;

said reset element comprising a pressure-responsive element arranged toapply a force to the balanceable device of the controller in accordancewith the pressure developed in said reset element by said adjustingmeans.

5. Apparatus as claimed in claim 4, wherein said adjusting meanscomprises a pneumatic force-balance transfer unit adapted to receive andcompare pressure signals corresponding to the secondary measurementsignal and said intermediate output signal, respectively, and to producea controlling pressure signal for said reset element of such a magnitudeas to a maintain equal said secondary measurement signal and saidintermediate output signal.

6. Apparatus as claimed in claim 5, wherein said transfer unit comprisesa planar element pivoted at an intermediate point and supplied onopposite surface on one side of said pivot point with pressurescorresponding to said secondary measurement signal and said intermediateoutput signal.

7. Apparatus as claimed in claim 1, wherein said secondary controllerincludes an activatable reset element to introduce reset action in theoutput signal from the secondary controller; and

adjusting means operable in independent mode and responsive to saidseparate signal for adjusting the activation of said reset element so asto maintain the output signal equal to said separate signal.

8. Apparatus as claimed in claim 7, wherein said secondary controller isof the pneumatic force-balance yp said reset element comprising apressure chamber adapted to receive a pressure signal to be applied tothe force-balanceable element of the controller in accordance with theseparate signal directed to said process operator element.

9. Apparatus as claimed in claim 8, wherein said secondary controllercomprises a laminar sandwich structure including a pivotally mountedplanar element serving as said force-balanceable element;

said sandwich structure including walls definin g pressure chambersadjacent respective sections of said planar element;

means to supply secondary measurement and secondary set pressure signalsto two of said chambers to produce oppositely-directed torques on .saidplanar element about its pivot axis;

position detector associated with said planar element to detect smallchanges in the position thereof responsive to unbalance in forcesthereon about said pivot axis; and

means controlled by said detector for adjusting the pressure in at leasta third chamber so as to maintain the torque, on said planar element inbalance.

10. Apparatus as claimed in claim 9, wherein said planar elementcomprises at least two separate segments on the same side of said pivotaxis, each of said segments being movable together about said axis inresponse to any unbalance in torques about said axis.

11. Apparatus as claimed in claim 10, wherein said sandwich walls definea pressure chamber adjacent one of said segments and separate from theother segment, whereby said one segment is individually activatable inaccordance with a respective pressure.

12. Apparatus as claimed in claim 11, including a seal along the pivotaxis to provide for independent pressures on opposite sides of the pivotaxis, said planar element extending through said seal and being pivotedabout said axis while maintaining the pressure integrity on oppositesides thereof.

13. Apparatus as claimed in claim 12, wherein said position detectorcomprises a pneumatic nozzle mounted adjacent said other segment tosense changes in the pivotal position of said planar element about saidaxis.

14. Apparatus as claimed in claim 13, wherein said other segment extendsout through said seal and beyond the exterior wall of said sandwichstructure, whereby said other segment and said nozzle are located inatmospheric pressure at all times.

15. Apparatus as claimed in claim 9, including further means controlledby said detector for adjusting the pressure in a fourth chamber inresponse to changes in the pivotal position of said planar element.

16. Apparatus as claimed in claim 15, wherein said further meansincludes means to alter the pressure in said fourth chamber in such amanner as to introduce reset action in the output signal of saidsecondary controller.

17. Apparatus as claimed in claim 16, wherein said reset introducingmeans includes an adjustable restrictor in the pressure line leading tosaid fourth chamber.

18. Apparatus as claimed in claim 17, wherein said first two chambersare located adjacent respective opposite surfaces of said planar elementwhich are on one side of said pivot axis; and

said other two chambers are located adjacent respective oppositesurfaces of said planar element which are on the other side of saidaxis;

said planar element being formed with an additional segment which moveswith the planar element about its pivot axis;

said additional segment being remote from any of said four pressurechambers so as not to be directly affected by the pressures therein; and

a position detector adjacent said additional segment to sense anyunbalance in forces about said pivot axis.

19. Apparatus as claimed in claim 8, wherein said adjusting meansincludes a non-restrictive conduit of relatively large flow capacity fordirecting a pressure signal to said reset pressure chamber correspondingto the Sigilfll being sent to the process operator element,

0. Apparatus as claimed in claim 19, wherein said transfer switch meansincludes a pneumaticallyoperated switch for opening said non-restrictiveconduit when the system is conditioned for non-automatic (independent)mode of operation.

21. In a cascade process control system of the type including a primarycontroller arranged to receive primary measurement and set point signalsand to provide an intermediate output signal to serve as a set pointsignal for a secondary controller arranged to receive a secondarymeasurement signal for comparison with the secondary set point signal,said secondary controller serving to provide an automatic control signalto adjust the setting of a process operator element such as a valve orthe like so as to maintain the primary measurement at its set pointvalue, and wherein the system is transferable between automatic andindependent (e.g. manual) control modes and for that purpose includes aselectively activatable means for producing a separate signal, in theform of a manual signal or the like, for controlling the processoperator element in place of said automatic control signal;

the method of conditioning the control system to prepare it forbumpless, balanceless transfer from independent mode to automatic modewhich comprises the steps of:

maintaining the automatic output signal of said secondary controllerequal to said separate signal; and

maintaining the intermediate output signal of said primary controllerequal to the secondary measurement signal without altering the primaryset point."

22. The method of claim 21, wherein said primary controller is of thereset type;

said intermediate output signal being maintained equal to said secondarymeasurement signal by adjusting the reset signal of said primarycontroller.

23. Themethod of claim 21, wherein said secondary controller is of thereset type;

said automatic output signal being maintained equal to said separatesignal by adjusting the level of the reset signal in said secondarycontroller.

24. The method of claim 23, wherein the reset signal in said secondarycontroller is maintained equal to the separate signal, and themeasurement and set signals of said secondary controller are maintainedequal.

25. The method of claim 24, wherein said primary controller is of thereset type;

said intermediate output signal and said secondary set point signalbeing maintained equal to said secondary measurement signal by adjustingthe level of reset activation in said primary controller.

26. The method of claim 21, wherein during automatic mode, said separatesignal is maintained equal to said automatic output signal to providefor bumpless, balanceless transfer to independent mode.

1. direct to the process operator element a control signal correspondingto said automatic output signal produced by said secondary controller(which automatic output signal had during the preceding independent modebeen maintained equal to said separate signal whereby the processoperator element will remain unaffected at the instant of switchback toautomatic mode); and
 1. direct to said process operator element acontrol signal corresponding to said separate signal;
 1. In a cascadeprocess control system of the type including a primary controllerarranged to receive both primary measurement and primary set pointsignals and to provide an intermediate output signal to serve as a setpoint signal for a secondary controller arranged to receive a secondarymeasurement signal for comparison with the secondary set point signal,said secondary controller serving to provide an automatic control signalto adjust the setting of a process operator element such as a valve orthe like so as to maintain the primary measurement at its set pointvalue, and wherein the system is transferable between automatic andindependent (e.g. manual) control modes and for that purpose includes aselectively activatable means operable in independent mode for producinga separate signal, in the form of a manual signal or the like, forcontrolling the process operator element in place of said automaticcontrol signal; that improvement in said control system for effectingbumpless, balanceless transfer from independent mode to automatic modewhich comprises, in combination: transfer switch means selectivelyoperable into automatic or independent status and serving in independentstatus to:
 1. In a cascade process control system of the type includinga primary controller arranged to receive both primary measurement andprimary set point signals and to provide an intermediate output signalto serve as a set point signal for a secondary controller arranged toreceive a secondary measurement signal for comparison with the secondaryset point signal, said secondary controller serving to provide anautomatic control signal to adjust the setting of a process operatorelement such as a valve or the like so as to maintain the primarymeasurement at its set point value, and wherein the system istransferable between automatic and independent (e.g. manual) controlmodes and for that purpose includes a selectively activatable meansoperable in independent mode for producing a separate signal, in theform of a manual signal or the like, for controlling the processoperator element in place of said automatic control signal; thatimprovement in said control system for effecting bumpless, balancelesstransfer from independent mode to automatic mode which comprises, incombination: transfer switch means selectively operable into automaticor independent status and serving in independent status to:
 1. direct tosaid process operator element a control signal corresponding to saidseparate signal;
 2. maintain said secondary controller functioning toproduce an automatic output signal corresponding to said separateindependent signal;
 3. maintain said primary controller functioning,with a constant set point, to produce an intermediate output signalcorresponding to the secondary measurement signal; said transfer switchmeans serving when switched back to automatic status to:
 1. direct tothe process operator element a control signal corresponding to saidautomatic output signal produced by said secondary controller (whichautomatic output signal had during the preceding independent mode beenmaintained equal to said separate signal whereby the process operatorelement will remain unaffected at the instant of switchback to automaticmode); and
 2. release said primary controller from the effectiveinfluence of said secondary measurement signal and return it to normaloperation under the influence of said primary measurement and setsignal, whereby the primary controller will not alter the functioning ofthe secondary controller so as to upset the process but will, afterswitchback to automatic, readjust the set point of the secondarycontroller to whatever extent is required for the purpose ofautomatically controlling the primary process condition to its setvalue.
 2. maintain said secondary controller functioning to produce anautomatic output signal corresponding to said separate independentsignal;
 2. Apparatus as claimed in claim 1, wherein said selectivelyactivatable means comprises a manual signal unit having means formanually controlling the magnitude of a signal for the process operatorelement.
 2. release said primary controller from the effective influenceof said secondary measurement signal and return it to normal operationunder the influence of said primary measurement and set signal, wherebythe primary controller will not alter the functioning of the secondarycontroller so as to upset the process but will, after switchback toautomatic, readjust the set point of the secondary controller towhatever extent is required for the purpose of automatically controllingthe primary process condition to its set value.
 3. maintain said primarycontroller functioning, with a constant set point, to produce anintermediate output signal corresponding to the secondary measurementsignal; said transfer switch means serving when switched back toautomatic status to:
 3. Apparatus as claimed in claim 1, wherein saidprimary controller includes a reset element to produce reset action inthe intermediate output signal; and adjusting means operable inindependent mode responsive to said secondary measurement signal foradjusting the activation of said reset element to maintain saidintermediate output signal equal to said secondary measurement signal.4. Apparatus as claimed in claim 3, wherein said primary and secondarycontrollers are of the pneumatic force-balance type; said reset elementcomprising a pressure-responsive element arranged to apply a force tothe balanceable device of the controller in accordance with the pressuredeveloped in said reset element by said adjusting means.
 5. Apparatus Asclaimed in claim 4, wherein said adjusting means comprises a pneumaticforce-balance transfer unit adapted to receive and compare pressuresignals corresponding to the secondary measurement signal and saidintermediate output signal, respectively, and to produce a controllingpressure signal for said reset element of such a magnitude as to amaintain equal said secondary measurement signal and said intermediateoutput signal.
 6. Apparatus as claimed in claim 5, wherein said transferunit comprises a planar element pivoted at an intermediate point andsupplied on opposite surface on one side of said pivot point withpressures corresponding to said secondary measurement signal and saidintermediate output signal.
 7. Apparatus as claimed in claim 1, whereinsaid secondary controller includes an activatable reset element tointroduce reset action in the output signal from the secondarycontroller; and adjusting means operable in independent mode andresponsive to said separate signal for adjusting the activation of saidreset element so as to maintain the output signal equal to said separatesignal.
 8. Apparatus as claimed in claim 7, wherein said secondarycontroller is of the pneumatic force-balance type; said reset elementcomprising a pressure chamber adapted to receive a pressure signal to beapplied to the force-balanceable element of the controller in accordancewith the separate signal directed to said process operator element. 9.Apparatus as claimed in claim 8, wherein said secondary controllercomprises a laminar sandwich structure including a pivotally mountedplanar element serving as said force-balanceable element; said sandwichstructure including walls defining pressure chambers adjacent respectivesections of said planar element; means to supply secondary measurementand secondary set pressure signals to two of said chambers to produceoppositely-directed torques on said planar element about its pivot axis;a position detector associated with said planar element to detect smallchanges in the position thereof responsive to unbalance in forcesthereon about said pivot axis; and means controlled by said detector foradjusting the pressure in at least a third chamber so as to maintain thetorque, on said planar element in balance.
 10. Apparatus as claimed inclaim 9, wherein said planar element comprises at least two separatesegments on the same side of said pivot axis, each of said segmentsbeing movable together about said axis in response to any unbalance intorques about said axis.
 11. Apparatus as claimed in claim 10, whereinsaid sandwich walls define a pressure chamber adjacent one of saidsegments and separate from the other segment, whereby said one segmentis individually activatable in accordance with a respective pressure.12. Apparatus as claimed in claim 11, including a seal along the pivotaxis to provide for independent pressures on opposite sides of the pivotaxis, said planar element extending through said seal and being pivotedabout said axis while maintaining the pressure integrity on oppositesides thereof.
 13. Apparatus as claimed in claim 12, wherein saidposition detector comprises a pneumatic nozzle mounted adjacent saidother segment to sense changes in the pivotal position of said planarelement about said axis.
 14. Apparatus as claimed in claim 13, whereinsaid other segment extends out through said seal and beyond the exteriorwall of said sandwich structure, whereby said other segment and saidnozzle are located in atmospheric pressure at all times.
 15. Apparatusas claimed in claim 9, including further means controlled by saiddetector for adjusting the pressure in a fourth chamber in response tochanges in the pivotal position of said planar element.
 16. Apparatus asclaimed in claim 15, wherein said further means includes means to alterthe pressure in said fourth chamber in such a manner as to introducereset action in the output signal of said secondary controller. 17.Apparatus as claimed in claim 16, wherein said reset introducing meansincludes an adjustable restrictor in the pressure line leading to saidfourth chamber.
 18. Apparatus as claimed in claim 17, wherein said firsttwo chambers are located adjacent respective opposite surfaces of saidplanar element which are on one side of said pivot axis; and said othertwo chambers are located adjacent respective opposite surfaces of saidplanar element which are on the other side of said axis; said planarelement being formed with an additional segment which moves with theplanar element about its pivot axis; said additional segment beingremote from any of said four pressure chambers so as not to be directlyaffected by the pressures therein; and a position detector adjacent saidadditional segment to sense any unbalance in forces about said pivotaxis.
 19. Apparatus as claimed in claim 8, wherein said adjusting meansincludes a non-restrictive conduit of relatively large flow capacity fordirecting a pressure signal to said reset pressure chamber correspondingto the signal being sent to the process operator element.
 20. Apparatusas claimed in claim 19, wherein said transfer switch means includes apneumatically-operated switch for opening said non-restrictive conduitwhen the system is conditioned for non-automatic (independent) mode ofoperation.
 21. In a cascade process control system of the type includinga primary controller arranged to receive primary measurement and setpoint signals and to provide an intermediate output signal to serve as aset point signal for a secondary controller arranged to receive asecondary measurement signal for comparison with the secondary set pointsignal, said secondary controller serving to provide an automaticcontrol signal to adjust the setting of a process operator element suchas a valve or the like so as to maintain the primary measurement at itsset point value, and wherein the system is transferable betweenautomatic and independent (e.g. manual) control modes and for thatpurpose includes a selectively activatable means for producing aseparate signal, in the form of a manual signal or the like, forcontrolling the process operator element in place of said automaticcontrol signal; the method of conditioning the control system to prepareit for bumpless, balanceless transfer from independent mode to automaticmode which comprises the steps of: maintaining the automatic outputsignal of said secondary controller equal to said separate signal; andmaintaining the intermediate output signal of said primary controllerequal to the secondary measurement signal without altering the primaryset point.
 22. The method of claim 21, wherein said primary controlleris of the reset type; said intermediate output signal being maintainedequal to said secondary measurement signal by adjusting the reset signalof said primary controller.
 23. The method of claim 21, wherein saidsecondary controller is of the reset type; said automatic output signalbeing maintained equal to said separate signal by adjusting the level ofthe reset signal in said secondary controller.
 24. The method of claim23, wherein the reset signal in said secondary controller is maintainedequal to the separate signal, and the measurement and set signals ofsaid secondary controller are maintained equal.
 25. The method of claim24, wherein said primary controller is of the reset type; saidintermediate output signal and said secondary set point signal beingmaintained equal to said secondary measurement signal by adjusting thelevel of reset activation in said primary controller.